1. Field of the Invention
The present invention relates to an internal combustion engine control signal processing system for signal processing for various kinds of control by using a knock signal of an internal combustion engine.
2. Related Art
A conventional internal combustion engine control signal processing system is disclosed in JP-A-6-159129. Much like the signal processing using a known vibration type knock sensor, in this system, a knock current having a specific frequency characteristic is separated from an ion current generated in the internal combustion engine by an analog circuit and integrated for the purpose of knock determination based on a result of the integration. When the amplitude of a knock signal appearing in an ion current decreases due to an operating condition or a noise component which is generated by an external source and superposed on the ion current, accurate knock determination using an analog circuit becomes difficult. Such a noise component is generated at a time a vehicle passes another vehicle or at a time the horn is operated.
In addition, another internal combustion engine control signal processing system is disclosed in JP-A-6-281523 is also known. In this system, an FFT (Fast Fourier Transform) frequency analysis is carried out for a specific range centered at a maximum amplitude waveform of a knock detection signal or a knocking waveform. As a result, processing can be carried out by a processor having a relatively low processing speed and the size of a memory used for the processing can be reduced. The knock detection signal attenuates gradually after its maximum amplitude. For this reason, the knock detection signal produced for the same time periods immediately before and after the maximum amplitude of the waveform are picked up for an analysis purpose. If the signal in the period immediately after the maximum amplitude is not available in the analysis due to a limitation on the size of the memory, accurate knock determination is impossible.
Also in the case of the internal combustion engine control signal processing system disclosed in JP-A-62-81523, the waveform of a knock detection signal is subjected to A/D-conversion with predetermined timing during a gate period. Values obtained as a result of the A/D-conversion are stored in a RAM. In the FFT (Fast Fourier transform) frequency analysis, typically 128 values stored in the RAM selected during a period in which a knock is actually generated are used. Since the number of values is small, the processing time can be shortened. If a knock center frequency or a sampling frequency is shifted, however, desired FFT processing values (spectrum intensities) can not be obtained.